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Flat belt mechanics

I don't remember seeing this before. Instead of baluster/bulbous pulley or capstan, two angled faces with a thin ridge in the centre on the drive pinion. So the belt must ride on the ridge but why does it not hunt about? There is a limiting flange either side of the angled flats and the width of the rubber band is greater than the distance between a flange and the central ridge, but still why no riding about?. Then what I thought was crude construction is probably part of the dynamics. The capstan is 2 discs fitted together but one is 49.94mm diameter and the other 50.04mm so this step change must make sure the band rides over the ridge but biased to one side, lightly touching one angled face, but not touching a limiting flange. When I get the mechanism working again I will try viewing with a xenon strobe light . Anyone any input on this ? more or less prone to changes in rubber restitution/stretch etc, more/less likely to come of when PTO is engaged, greater/less accuracy in speed of drive with varying driven-side back torque etc.

Reply to
N_Cook
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I don't know what the simple explanation is, but flat belts will typically self-center on a crown or high spot on a wheel, spindle etc.

This same crown effect is used on many machines, not just tape drive mechnisms, where they're often seen in electronic repair, but as you mention not usually made up of flat angles.

-- Cheers, WB .............

Reply to
Wild_Bill

mention

or

about?

When

engaged,

I just looked in my box of salveage motor spindle pinions, perhaps 30 are bulbous but no flatted/double-cone ones with this central ridge in there I always assumed the curvature of a bulbous pinion was something a bit tighter than the natural curve of the original flat belt if pressured in the centre only. So if a ridge rather than a soft curved high spot then perhaps functionally much the same. While at it , IIRC bulbous edged capstan and flat drive pinion or bulbous pinion and flat edged capstan, never both bulbous or you will get a hunting effect coming in

Reply to
N_Cook

Thats just reminded me of the DAF Variomatic automatic gearbox. Here's one working on Youtube.

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Gareth.

Reply to
Gareth Magennis

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Oops, wrong link. This one works:

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Gareth.

Reply to
Gareth Magennis

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At one time Philips washing machines used a very similar system.

Ron

Reply to
Ron

Useful to remember if no machine shop available for making a bulbous pulley, if faced with replacing a motor and its the wrong speed range so forced change of pulley , or smaller spindle diameter. Except the ridge is semi-circular profile here and bodge job would be chamfered edges but starting from a brass rod it would be a lot easier to fashion leaving central ridge, bodging 2 cone segments and 2 end faces , than fully bulbous without a lathe

Reply to
N_Cook

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Well I'll go to the foot of our stairs!

Gareth.

Reply to
Gareth Magennis

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Aye lad, they used a plastic[1] pulley containing three ball bearings fert centrifugal whatnot. If the drum got jammed, the belt would slip, the pulley would burn and emit the smell most horrible.

[1] Looked like Bakelite but probably wasn't

Ron

Reply to
Ron

pulley

drive

the

was

discs

step

one

flange.

strobe

back

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Dyson vacuum cleaners have a similar mechanism. An unnecessary football rattle mechanism in the slip clutch so a disturbingly nasty noise on a jam. Incidently anyone else find the phrase "No loss of suction" for a system that forces air down draggy 10 foot of narrow piping, inside the machine, unlike conventional ones- air in and out over 1 foot if that.

Reply to
N_Cook

pulley,

bulbous

Back working with a silicone rubber flat belt, shame it is bright orange. Looking under x30 the original band had a furrow along its centre. I've since found a salvaged variant on this pinion , 2 angled cones with an included angle of about 110 degrees but no ridge , just a rounded join probably about .5mm diameter again. The band cannot deflect that much so must ride on that central edge, like the one I just fixed. Perhaps bulbous ones are just kinder on the rubber

Reply to
N_Cook

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